Gate BuiltinImporter and FrozenImporter load_module to Python < 3.15

[raul-sq]

load_module() was removed in Python 3.15 (deprecated since Python 3.4;
see the 3.15 What's New / importlib docs). This gates the declarations on
_frozen_importlib.BuiltinImporter and _frozen_importlib.FrozenImporter
behind sys.version_info < (3, 15) with a @deprecated message, following
the existing find_module pattern in the same file.

Addresses two TODO entries in stdlib/@tests/stubtest_allowlists/py315.txt
(entries intentionally left in place for now; will remove in a follow-up
commit once CI confirms).

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[AlexWaygood]

Thank you!

[AlexWaygood]

Interesting -- there's no deprecation warning at runtime:

% uvx python3.14
Python 3.14.5 (main, May 10 2026, 19:20:57) [Clang 22.1.3 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from _frozen_importlib import *
>>> BuiltinImporter.load_module('abc')
<module 'abc' (built-in)>
>>> FrozenImporter.load_module('abc')
<module 'abc' (frozen)>
>>> FrozenImporter.load_module('shutil')

but I do see the general deprecation notice at https://docs.python.org/3.12/deprecations/index.html#pending-removal-in-python-3-15, so I think this is an appropriate use of @deprecated.

I feel like your existing comment doesn't add that much over what you can see in the code. Maybe get rid of it and replace it with a note that all load_module methods in importlib were deprecated, and link to https://docs.python.org/3.12/deprecations/index.html#pending-removal-in-python-3-15 ?